Microscratches are scratches that are on the order of several microns in width and submicron to microns in depth. They are commonly observed on the front and back sides of photographic films, on photoconductor belts, on thermal prints, and on PhotoCD disks. They are caused by sliding contact of imaging products with dirt particles or other asperities that have micron-sized contact radii. These scratches can affect analog or digital image transfer and degrade the output image quality. Their presence on magnetic or conductive backings could lessen the performance of these functional coatings. Thus, scratch resistance protective coatings on the front or back or both sides of an imaging product are commonly required.
Since all imaging products are based on flexible substrates for ease of transport, conveyance, and manufacturing, hard metallic or ceramic tribological scratch resistant coatings are not suitable due to their mechanical incompatibility with the polymeric flexible substrates. This mechanical incompatibility can cause adhesion failure between the coating and the substrate during scratching. Polymeric coatings are thus preferable as the scratch resistant layer for imaging products. However, with the requirements for high light transmission, low material cost, low internal drying stress, and high coating speeds, the thickness of these scratch resistant coatings is preferably about 10 microns or less.
During micro-scratching of a micron-thick coating, complex stress fields develop in the coating, within which high internal shear stress, interfacial shear stress, and surface tensile stress are present. A coating can fail either by shear fracture, delamination, or tensile cracking depending on the relative shear, adhesive, and tensile strengths of the coating. Using a micro-scratching instrument with a single micron-sized stylus, the resistance to scratch damage for a coating can be measured. Combining this instrument with optical microscopy, the failure mode, such as shear fracture, delamination, or tensile cracking, can be determined. All these failure modes produce scratches that are printable and scanable and, thus, unacceptable for imaging products. A permanent scratch track resulting from plastic deformation of a ductile coating without coating failure is also printable and scanable, and thus, not desirable.
Various types of polymeric coatings have been examined as scratch resistant coatings for imaging products. These include coatings comprising brittle, ductile, elastic-plastic, or rubber-elastic polymeric materials. Brittle polymers with elongations to break less than 5%, such as poly(methyl methacrylate) and poly(styrene) are not desirable as scratch resistant coatings for imaging products. Regardless of the coating thickness, the brittleness of these materials leads to printable surface tensile cracks during scratching. Soft elastomers (rubber-elastic materials), such as urethane rubbers, acrylic rubbers, silicone rubbers, are not suitable as scratch resistant coatings since deep penetration of the asperity or stylus occurs in these soft coatings which causes these elastomeric coatings to fail at low loads during scratching. Using stiff fillers to increase the stiffness of these elastomers to reduce stylus penetration does not solve this problem since permanent and printable scratch tracks result in elastomeric coatings containing stiff fillers by the induced coating plasticity under the presence of stiff fillers.
Ductile elastic-plastic coatings with elongations to break greater than 10%, such as glassy polyurethanes, polycarbonate, cellulose esters, etc., exhibit shear-fracture-type scratch damage during scratching that result from plastic flow. Plastic flow in these ductile coatings during scratching is controlled by the coating thickness. For thin coatings of these materials, plastic flow in the coating during scratching is restricted by the coating adhesion to the substrate leading to a premature failure of the coatings at low loads. Thicker coatings for these materials may have improved resistance to coating failure, however, for imaging products these thicknesses may be impractical. In addition, although thick ductile coatings have improved resistance to coating failure during scratching, the low yield strength and modulus for these materials result in the formation of permanent scratch tracks in the coatings at low loads.
It can be seen that various approaches have been attempted to obtain an improved scratch resistant layer for imaging products. However, the aforementioned methods have met with only limited success. Recently, in commonly-assigned U.S. Ser. No. 09/089,794 a coating composition is disclosed with resistance to the formation of permanent scratch tracks and coating failure when an imaging product is exposed to sharp asperities or other conditions that may lead to scratches during the manufacture and use of the imaging product. However, such a backing does not necessarily provide any antistatic characteristics required of an imaging element for its successful manufacture, finishing and subsequent use. Although a number of oxides with electronic conductivity have been proposed as stiff fillers in U.S. Ser. No. 09/089,794, their inclusion at the proposed dry volume fraction and coverage is likely to impart unacceptable levels of color and haze to the photographic element. Moreover, due to the highly filled nature of such a backing, it cannot be used as a barrier layer, against photographic processing solutions, over vanadium oxide based antistats disclosed in U.S. Pat. 5,679,505 and references therein and, hence, will not insure "process-surviving" conductivity of such antistats. The present invention is intended to provide improved scratch resistance and antistatic properties, before and after film processing, all in a single layer with acceptable optical properties for application in imaging elements.
The problem of controlling static charge is well known in the field of photography. The accumulation of charge on film or paper surfaces leads to the attraction of dirt which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or "static marks" in the emulsion. The static problems have been aggravated by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the finished photographic film product. In an automatic camera, the winding of roll film in and out of the film cartridge, especially in a low relative humidity environment, can result in static charging. Similarly, high speed automated film processing can result in static charge generation. Sheet films (e.g., x-ray films) are especially susceptible to static charging during removal from light-tight packaging.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive "antistatic" layers into the film structure. Antistatic layers can be applied to one or to both sides of the film base as subbing layers either beneath or on the side opposite to the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivity. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during photographic processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility rather than ionic mobility and is independent of humidity.
Antistatic layers containing electronic conductors such as conjugated conducting polymers, conducting carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconducting thin films can be used more effectively than ionic conductors to dissipate static charge since their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors, electrically conducting metal-containing particles, such as semiconducting metal oxides, are particularly effective when dispersed in suitable polymeric film-forming binders in combination with polymeric non-film-forming particles as described in U.S. Pat. Nos. 5,340,676; 5,466,567; 5,700,623. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example, U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; 5,484,694 and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include antimony-doped tin oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, and niobium-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other conductive metal-containing granular particles including metal borides, carbides, nitrides and suicides have been disclosed in Japanese Kokai No. JP 04-055,492.
One serious deficiency of such granular electronic conductor materials is that, especially in the case of semiconductive metal-containing particles, the particles usually are highly colored which render them unsuitable for use in coated layers on many photographic supports, particularly at high dry weight coverage. This deficiency can be overcome by using composite conductive particles consisting of a thin layer of conductive metal-containing particles deposited onto the surface of non-conducting transparent core particles whereby obtaining a lightly colored material with sufficient conductivity. For example, composite conductive particles consisting of two dimensional networks of fine antimony-doped tin oxide crystallites in association with amorphous silica deposited on the surface of much larger, non-conducting metal oxide particles (e.g., silica, titania, etc.) and a method for their preparation are disclosed in U.S. Pat. Nos. 5,350,448; 5,585,037 and 5,628,932. Alternatively, metal-containing conductive materials, including composite conducting particles, with high aspect ratio can be used to obtain conducting coatings with lighter color due to reduced dry weight coverage (vide, for example, U.S. Pat. Nos. 4,880,703 and 5,273,822). However, there is difficulty in the preparation of conductive coatings containing composite conductive particles, especially the ones with high aspect ratio, since the dispersion of these particles in an aqueous vehicle using conventional wet milling dispersion techniques and traditional steel or ceramic milling media often result in wear of the thin conducting layer from the core particle and/or reduction of the aspect ratio. Fragile composite conductive particles often cannot be dispersed effectively because of limitations on milling intensity and duration dictated by the need to minimize degradation of the morphology and electrical properties as well as the introduction of attrition-related contamination from the dispersion process.
Electrically-conductive layers are also commonly used in imaging elements for purposes other than providing static protection. Thus, for example, in electrostatographic imaging it is well known to utilize imaging elements comprising a support, an electrically-conductive layer that serves as an electrode, and a photoconductive layer that serves as the image-forming layer. Electrically-conductive agents utilized as antistatic agents in photographic silver halide imaging elements are often also useful in the electrode layer of electrostatographic imaging elements.
As indicated above, the prior art on electrically-conductive layers in imaging elements is extensive and a very wide variety of different materials have been proposed for use as the electrically-conductive agent. There is still, however, a critical need in the art for improved electrically-conductive layers which are useful in a wide variety of imaging elements, which can be manufactured at reasonable cost, which are environmentally benign, which are durable and scratch-resistant, which are adaptable to use with transparent imaging elements, which do not exhibit adverse sensitometric or photographic effects, and which maintain electrical conductivity even after coming in contact with processing solutions (since it has been observed in industry that loss of electrical conductivity after processing may increase dirt attraction to processed films which, when printed, may cause undesirable defects on the prints).
It is towards the objective of providing a scratch-resistant, antistatic layer for imaging elements especially for silver halide photographic films that survives film processing that the present invention is directed. The layer of the present invention comprises in particular a specific ductile polymer, a hard filler and an electrically conducting polymer.
Electrically conducting polymers have recently received attention from various industries because of their electronic conductivity. Although many of these polymers are highly colored and are less suited for photographic applications, some of these electrically conducting polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 5,300,575; 5,312,681; 5,354,613; 5,370,981; 5,372,924; 5,391,472; 5,403,467; 5,443,944; 5,575,898; 4,987,042 and 4,731,408) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550; 5,093,439 and 4,070,189) are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity instead of ionic conductivity, these polymers are conducting even at relative humidity as low as 5%, as demonstrated in copending applications U.S. Ser. Nos. 09/173,409 and 09/172,878. Moreover, these polymers can retain sufficient conductivity even after wet chemical processing to provide what is known in the art as "process-surviving" antistatic characteristics to the photographic support they are applied to, as also demonstrated in copending applications U.S. Ser. Nos. 09/173,409 and 09/172,878. Unlike metal-containing semiconducting particulate antistatic materials (e.g., antimony-doped tin oxide), the aforementioned electrically conducting polymers are less abrasive, environmentally more acceptable (due to absence of heavy metals), and, in general, less expensive.
However, it has been reported (U.S. Pat. No. 5,354,613) that the mechanical strength of a thiophene-containing polymer layer is not sufficient and can be easily damaged without an overcoat. Protective layers such as poly(methyl methacrylate) can be applied on such thiophene-containing antistat layers but these protective layers typically are coated out of organic solvents and therefore not highly desired. More over, these protective layers may be too brittle to be an external layer for certain applications, such as motion picture print films (as illustrated in U.S. Pat. No. 5,679,505). Use of aqueous polymer dispersions (such as vinylidene chloride, styrene, acrylonitrile, alkyl acrylates and alkyl methacrylates) has been taught in U.S. Pat. No. 5,312,681 as an overlying barrier layer for thiophene-containing antistat layers, and onto the said overlying barrier layer is adhered a hydrophilic colloid-containing layer. But, again, the physical properties of these barrier layers may preclude their use as an outermost layer in certain applications. The use of a thiophene-containing outermost antistat layer has been taught in U.S. Pat. No. 5,354,613 wherein a hydrophobic polymer with high glass transition temperature is incorporated in the antistat layer. But these hydrophobic polymers reportedly may require organic solvent(s) and/or swelling agent(s) "in an amount of at least 50% by weight," for coherence and film forming capability.
As will be demonstrated hereinbelow, the present invention provides a scratch resistant antistatic layer comprising a specific ductile polymer, a hard filler and an electrically conducting polymer which provides certain advantages over the teachings of the prior art including the retention of antistatic properties after color photographic processing.